1. Field of the Invention
This invention is concerned with an acousto-optical seismic sensor array for use in a seismic marine streamer cable as used for seismic exploration.
2. Discussion of the Prior Art
In marine seismic exploration, a long streamer cable, containing many hundred pressure sensors or hydrophones, is towed through the water at an assigned depth. At intervals, as a towing ship steams along a line of survey, a sound source emits an acoustic signal or shot. The resulting acoustic waves propagate downward through the sub-bottom earth layers whence they are reflected back to the water surface. The reflected waves are sensed by the hydrophones as water-pressure variations which are converted to analog electrical signals. The signals from the hydrophones are transmitted by wire lines or coaxial cables to the towing ship for use by a signal utilization device. As a matter of interest, the seismic signals have a wide dynamic range of at least 120 dB. The overall sensitivity of the pressure sensors and the ancillary signal processing system is therefore an important consideration.
Conventional electrical data-gathering systems in current use are often of a telemetric design where one or two broad-band transmission lines link the hydrophones to the shipboard data-processing equipment. Each subgroup of hydrophones (of which there may be 50 or more) requires electrically active multiplexers and repeaters. Thus, not only signal transmission lines but also electrical power lines are required to be included in the streamer cable. Being electrical in nature, signals from one sensor channel are subject to crossfeed into some other channel due to electrical leakage. Generally the pressure sensors in common use are piezoelectric crystals which produce a very small signal and are characterized by a very high impedance. Impedance-matching networks are required for each sensor or group of sensors.
Optical-fiber technology has been investigated to replace the traditional hard-wired/electrical technology presently in use for seismic streamer cables. Optical components are hardened against cross-fed electrical transients and may be passive systems.
It is well known that changes in ambient pressure cause changes in length and index of refraction of an optical fiber. Such environmental changes will cause changes in the intensity, phase, and polarization of a light pulse propagating through the fiber. The incremental change in the light-pulse characteristics per unit of fiber length is small. Accordingly the sensor portion of an acousto-optical data acquisition system usually consists of a small coil of optical fiber wound around a mandrel. The coil is then exposed to the medium whose physical parameters are to be measured.
Typically, the sensing fiber or coil is used in conjunction with a reference fiber mounted in a controlled environment. A laser-launched light beam is transmitted in parallel through both the sensor fiber and the reference fiber. The relative difference in intensity, phase shift or polarization angle of the output light is a direct function of the quantity being measured.
Most acousto-optical devices employ a pair of photo diodes, with or without an interferometer, as a light-beam comparator device. In a laboratory environment, the comparator device as well as the reference fiber, which are relatively delicate, remain under controlled conditions while the sensor itself is used as a probe mounted on the end of a suitable optical-fiber cable for limited excursions to remote locations. For seismic operations, such a relatively simple configuration is not possible.
A seismic streamer cable for marine use is a distributed-sensor data-acquisition system. Several hundred sensors are employed over several kilometers. In some suggested optical systems, the reference fiber and comparator device such as an interferometer are co-located with the sensor itself. Data are returned to a processing system on board the ship either by a multiple-fiber cable or over a single-fiber time-division or wavelength-division multiplexing system. The disadvantage here is the need for active devices in the seismic cable with consequent power-supply complications. Additionally, the reference fibers and comparators are subjected to an undesirable hostile environment.
Alternate arrangements have been suggested for use with a passive acousto-optical system. The system requires two optical fibers, one for feeding laser pulses to a plurality of optical-fiber sensors and one for returning the acoustic-signal-modulated light beam to the towing ship. The inputs of the plurality of optical-fiber sensors are coupled in parallel to the feed line by suitable well-known directional optical couplers. The outputs of the sensors are similarly coupled in parallel to the return line. A single absolute-reference fiber is coupled between the feed line and the return line in conjunction with a light-comparator device. As a laser pulse propagates through the feed line, a portion of the light is tapped off to each of the respective sensors. The tapped-off light pulse then returns to the absolute-reference/comparator network as a stream of pulses in time-division-multiplexed format. Of course, the original pulse is also transmitted by the laser to the absolute reference/comparator network through a tapped optical delay line. The tapped delay line is required to compensate for the increasing distances of the sensors from the ship and the consequent pulse-time delays.
There are many disadvantages in the above system. There are several hundred sensors, each of which requires two optical couplers per sensor. Commercially-available couplers cost several hundred dollars each so that such a system is costly. The tapped delay line requires as many taps as there are sensors as well as some form of active multiplexer to synchronize the delayed reference pulse with the returning data-signal-modulated pulses. Furthermore cumulative errors in the physical dimensions of 3- or 4-Kilometer streamer cables would make it difficult to provide an accurately-tapped delay line.
It is an object of this invention to provide an economical, simple acousto-optical system having a minimal number of components, for use in seismic exploration.